In recent years, SSDs (Solid State Drives) using a semiconductor device as a storage medium have been increasingly used in storage systems. SSDs are superior in random access performance to conventional HDDs. Schemes to develop SSDs larger in capacity and lower in price as SSDs using NAND flash memories in particular have progressed and the replacement of HDDs with such SSDs is now in progress. SCMs (Storage Class Memories) as a nonvolatile storage medium are smaller in capacity and higher in price than NAND flash memories but have higher access speeds and can be rewritten a larger number of times and are, therefore, attracting attention as a high-performance long-life recording medium. SCMs are a kind of nonvolatile semiconductor memory, e.g., an MRAM (Magnetic Random Access Memory), a PRAM (Phase Change Random Access Memory) and a ReRAM (Resistance Random Access Memory).
The system performance of a storage system is improved by utilizing unevenness of the frequency of access to data and by providing a storage controller with a cache using a high-speed storage medium. This is because a large overhead occurs if all possible accesses to an HDD that is a final storage place for data are made. If data frequently accessed is stored in a cache, an access can be made at a high speed to the data without accessing the HDD when the data is again accessed. Accordingly, if the cache capacity is increased, the amount of data accessible at a high speed is increased. That is, the rate of hitting data in the cache is improved and an improvement in the system performance can be expected.
DRAMs are being widely used as caches in storage systems. DRAMs are much higher in speed than HDDs but have problems in terms of increasing the capacity and in terms of price. In recent years, techniques using an SSD having lower in price and having a characteristic of a larger capacity in comparison with DRAMs as a cache in a storage system have been disclosed. An SSD using NAND flash memories lower in price and having a large capacity in comparison with DRAMs is used as a secondary cache in a storage system to increase the cache hit rate and improve the system performance and the cost performance.